Two-cycle engines, such as those used in vehicles, are generally designed to operate in one of two distinct engine speed, or revolutions per minute (RPM), regions. Thus, the prior art engine may be designed to operate in a low-engine-speed region or a high-engine-speed region. A prior art engine designed to operate in a low-engine-speed region has a peak power output only in this low-engine-speed region. Likewise, a prior art engine designed to operate in a high-engine-speed region has peak power output only in this high-engine-speed region. Thus, a prior art engine does not have a peak power output at both a low engine speed and at a high engine speed.
In contrast, the present invention raises the power output of a two-cycle engine at the low and middle engine-speed regions, and maintains a high peak power output at the high-engine-speed region. Thus, the present invention allows a two-cycle engine to operate at relatively high power outputs at all three of the low, middle and high engine speed regions.
One attempt at optimizing the performance of a two-cycle engine at different operating speeds involves adjusting the top of the opening of the exhaust port of the two-cycle engine between two predetermined positions, as desired by the operator. Thus, FIG. 1 of U.S. Pat. No. 4,516,540 shows a movable plate with an arc-like edge. In operation, the arc-like edge may be positioned to adjust the top of the exhaust port between either one of two distinct positions. However, such attempts to optimize the performance of a two-cycle engine ignore the fact that in a low-engine-speed region, while the compression ratio and the torque of the engine are very sensitive to the position of the top of the exhaust port, the extent to which the combustion products are exhausted and the fresh air-fuel mixture is introduced into the engine's combustion chamber are also very sensitive to the position of the top of the exhaust port.
That is, if the top of the exhaust port in a two-cycle engine is lowered in order to delay the exhaust timing, then the start of the compression stroke is accelerated to the same extent that the start of the exhaust stoke is delayed. This thereby increases the compression and the torque of the engine at low engine speeds. At the same time, the exchange rate of the fresh air-fuel mixture is lowered by lowering the top of the exhaust port. This has the effect of causing irregular combustion in the low-engine-speed region. Therefore, the desired result of optimizing the engine performance in the low-engine-speed region is not accomplished.
In an attempt to overcome the problems associated with an exhaust port opening that may be set at only one of two positions, the inertial mass of the flywheel of some previous two-cycle engines has been increased. However, this increase in the inertial mass of the flywheel has a tendency to decrease the responsiveness of the engine. Thus, the desired result of increased engine performance in a variety of engine operating speeds again is not attained.